It is common to isolate many different types of gas process equipment for such purposes as maintenance, inspection, etc. When such isolation occurs or is desired, dampers in the gas flow path are actuated to seal or isolate this process equipment from the main process gas flow path. Oftentimes, the diverted flow travels through a by-pass or it can be sent to other parallel process equipment or such gas flow can be decreased, or is already low, so as to accommodate the removal of the isolated equipment from operation.
Upon such isolation, seal air fans are normally activated to pressurize the isolated equipment thereby creating a positive pressure difference across the seal dampers. This insures that no leakage into the isolated equipment will occur. Generally, the design seal air pressure in such an isolated system is typically set above the highest possible operating pressure obtainable in the main process gas flow path to insure that no leakage into the isolated system will occur. Normally, however, the actual pressure within the main process gas flow path will be considerably less than this highest possible operating pressure thereby resulting in a vast pressure difference across the closed dampers due to the even higher seal air pressure. It is this pressure difference which must be overcome whenever the dampers are to be opened and the process equipment is to be brought back on-line.
For example, it is not uncommon for gas process equipment to have a maximum operating pressure of 15 inches water gauge thereby warranting a seal air pressure of about 20 inches water gauge after upsets and other factors are taken into consideration. However, during periods of low demand when it is likely that a module will be removed from service, the actual operating pressure in the main gas flow path can, in reality, be as low as 1 inch water gauge. Thus, the resulting pressure differential across the dampers between the actual low main gas flow path operating pressure and the high seal air pressure will be as high as 19 inches water gauge. This is a large pressure difference that must unnecessarily be maintained by the dampers when, in fact, a much lower pressure difference, such as in the 4 to 8 inch water gauge range more or less, will suffice to maintain a proper seal. Furthermore, when the time comes to open the dampers, they must push or operate against this large seal air pressure thereby requiring larger and stronger dampers and related motors and/or drivers.
In other systems, the maximum operating pressure of the main gas flow path can be in the 20-30 inches of water gauge range thereby warranting an even higher seal pressure when, in fact, the actual operating pressure may, in reality, be considerably less.
Because such isolated systems are to be pressurized, it is common to install an overpressurization relief valve to prevent the isolated module from becoming too pressurized. Such a relief valve is of no use in the present situation since these relief valves are designed to operate only upon reaching or exceeding a preset maximum pressure value, they are not designed to limit or maintain the seal air pressure to only a few inches water gauge above or relative to the pressure on the other or main gas flow path side of the damper.
Thus, these overpressurization relief valves merely insure that the module's design pressure is not exceeded by the operation of the seal fans. They are configured so as not to open until overpressurization occurs and their vents are sized to hold the module at the designated seal air pressure if need be.
It is thus an object of the present invention to provide a means of maintaining a pressure difference across the closed seal dampers of only a few inches water gauge, say about 4 to 8 inches water gauge, more or less. While it is not expected that this pressure difference will vary, the actual value of the seal air pressure may vary as this value is a function of the actual operating pressure in the main gas flow path. It is another object of this invention to prevent overpressurization of the isolated module from occurring and to release such pressure if overpressurization does, indeed, occur. Still another object of the present invention is to provide a means of reducing the pressure of the isolated module prior to damper operation. Yet another object of the present invention is to reduce the size and power requirements of such isolation dampers. These and other objects and advantages of the present invention will become obvious upon further investigation.